1. Field
The disclosure relates to an organic compound and organic electroluminescence device employing the same and, more particularly, to an organic compound serving as a host material and a phosphorescent organic electroluminescence device employing the same.
2. Description of the Related Art
Recently, with the development and wide application of electronic products, such as mobile phones, PDAs, and notebook computers, there has been increasing demand of flat display elements which consume less electric power and occupy less space. Organic electroluminescent devices are self-emitting and highly luminous, with wider viewing angles, faster response speeds, and simpler fabrication methods, making them an industry display of choice.
Generally, an organic electroluminescent device was composed of a light-emission layer sandwiched between a pair of electrodes. When an electric field was applied to the electrodes, the cathode injects electrons into the light-emission layer and the anode injects holes into the light-emission layer. When the electrons recombination with the holes in the light-emission layer, excitons were formed. Recombination of the electron and hole results in light emission.
Depending on the spin states of the hole and electron, the exciton which results from the hole and electron recombination can have either a triplet or singlet spin state. Luminescence from a singlet exciton results in fluorescence whereas luminescence from a triplet exciton results in phosphorescence. The emissive efficiency of phosphorescence is three times that of fluorescence. Therefore, it is crucial to develop highly efficient phosphorescent materials, in order to increase the emissive efficiency of the OLED.
In application of organic electroluminescent devices, phosphorescent guest materials have to be used in combination with host materials which has an energy gap matched therewith, thereby achieving optimal electroluminescent performance and quantum yield. Particularly, since blue and green host materials require larger differences of energy gap between the host and guest materials for electroluminescence, the host materials used in an phosphorescent OLED should have a shorter conjugated system. Further, in order to keep the key characteristics of the organic compound used in OLEDs (i.e. thermal-stability), the host material should also have larger molecular weight, resulting in difficulties for chemical structure designs.
Since conventional, commercially available phosphorescent host materials or phosphorescent host materials disclosed in prior art references merely have the moieties of carbazole or silyl benzene derivatives, the phosphorescent host materials exhibit inferior thermal stability, resulting in devices made therefrom to have low current density and high operating voltage.